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PHOENIX -- A new study adds to years of research showing that childhood vaccines do not cause autism, despite worries among a growing number of parents that their young children receive "too many vaccines."

"Jalen has pretty severe autism," dad Jami Nielsen said of his son. "When they were 3, both boys went to get shots and within two to three days, all his language fell completely off."

That's when Jalen, now 10, was diagnosed as autistic. His parents always wondered if the shots were the culprit.

"You know, that's crossed our mind. We do have a younger daughter we opted not to give her any of the shots yet and she's 6," Nielsen said. "We feel there is some correlation but we've also wondered if there's a precursor for autism maybe that triggers it."

It's the reason why, according to the Centers for Disease Control, one in 10 parents refuse or delay their child's vaccinations. A primary concern is the number of vaccinations administered both in a single day and in the child's first two years. But a new study by the CDC finds no link between too many vaccines and autism.

Dr. Frank DeStefano and colleagues from the CDC and Abt Associates, Inc. analyzed data from 256 children with autism spectrum disorders (ASDs) and 752 children without ASD (born between 1994 and 1999) from three managed-care organizations. They looked at each child's cumulative exposure to antigens, the substances in vaccines that cause the body's immune system to produce antibodies to fight disease, and the maximum number of antigens each child received in a single day of vaccination.

The researchers determined the total antigen numbers by adding the number of different antigens in all vaccines each child received in one day, as well as all vaccines each child received up to 2 years old. The authors found that the total antigens from vaccines received by age 2, or the maximum number received on a single day, was the same between children with and without ASDs, which are defined by the CDC as "a group of developmental disabilities that can cause significant social, communication and behavioral challenges."

Furthermore, when comparing antigen numbers, no relationship was found when they evaluated the sub-categories of autistic disorder and ASDs with regression.

Although the current routine childhood vaccine schedule contains more vaccines than the schedule in the late 1990s, the maximum number of antigens that a child could be exposed to by 2 years of age in 2013 is 315, compared with several thousand in the late 1990s.

Because different types of vaccines contain varying amounts of antigens, this research acknowledged that merely counting the number of vaccines received does not adequately account for how different vaccines and vaccine combinations stimulate the immune system. For example, the older whole-cell pertussis vaccine causes the production of about 3,000 different antibodies, whereas the newer acellular pertussis vaccine causes the production of six or fewer different antibodies.

"We did not find any relationship between the number of antigens and the risk of autism," DeStefano said.

Still there has been a recent spike in the number of children with autism. Some report the number as high as one in 50.

"That may be due to better diagnosis, better awareness, better recognition, different diagnostic criteria and different ways we treat it," said Christopher Smith, vice president of the Southwest Autism Research and Resource Center.

And Nielsen still wonders about that last vaccine his son had, he's still not certain it's to blame.

"No, because I've seen too many other things out there," he said.

And while the experts at SARRC don't know what causes autism, they don't believe it's vaccinations and suggest parents speak to their children's doctors.

"Our position is also that it is really important people vaccinate their kids because the risk of these other diseases that we can protect against is much greater," Smith said. "By and large the most important thing is to vaccinate your child."